Abstract : Recent published similarity solutions for spherically and cylindrically symmetric gas expansions in rarefied atmospheres are extended to include the influence of ambient pressure. The former solutions and extended solutions are then compared with exact numerical calculations for the same gas expansions. Limits of applicability of each solution are discussed, and results compared with observations for typical barium gas release in the upper atmosphere. The extended similarity solutions have a range of validity up to an order of magnitude greater than that of the previous solutions. However, all of the similarty solutions are rendered inapplicable by the early onset of diffusion as the dominate expansion mechanism. Apparent agreement at later times is due to the similar growth rates for diffusion about t to the 0.5 power and similarity about t to the 0.4 power. (Author)